Birefringent-filter-assisted actively Q-switched tuneable erbium-doped fibre ring laser

We report the performance of a narrow-linewidth, actively Q-switched erbium-doped fibre ring laser emitting at ~1.55μm. An investigation into the behaviour of a cascaded comb and tuneable filter system was carried out using a length of polarisation-maintaining fibre in a Lyot filter configuration. The resulting comb filter was used to suppress competing modes, and the addition of a tuneable filter allowed wavelength tuning over 33nm. Proper adjustment of the spectral overlap of the cascaded filters allowed the linewidth to be reduced to 65pm (8GHz). Q-switching was achieved using an acousto-optic modulator, providing stable pulses at repetition frequencies between 9kHz and 166kHz, with a minimum pulse length of 400ns. These results demonstrate the feasibility of such a system for use in a narrow linewidth, tuneable laser.